There is provided a method of testing a first device using a tester. The method includes receiving test data having a pattern by the first device from the tester; detecting the pattern of the test data by the first device; generating first data, by the first device, according to the pattern detected by the detecting; comparing the test data with the pattern detected by the detecting; determining errors in the test data, by the first device, based on the comparing; inserting the errors into the first data to generate error-inserted first data; and transmitting the error-inserted first data by the first device to the tester. The method may further include generating a first clock at the first device; wherein the transmitting uses the first clock for transmitting the error-inserted first data.